In preliminary studies of a random collection of cDNA clones representing cytoplasmic polyA+ RNA from rat brain, we have found that roughly 50% of the mRNAs in brain are brain specific and the levels of another 30% of the mRNA molecules are differentially regulated in brain, liver and kidney tissues. The average length of brain specific mRNAs (3300 nucleotides) is considerably larger than the average length of regulated, non-specific mRNAs (2500 nucleotides) which is again considerably larger than the average length of unregulated mRNAs (1800 nucleotides). Ten percent of the clones hybridize to 2 or 3 mRNAs and these seem to be coordinately expressed in each instance. Several of the cDNA clones (3%) contain a common 82 nucleotide "ID" sequence followed by an oligo dA tract. This common sequence, present in the genome in 1.5 x 10 to the 5 copies, hybridizes to an abundant brain specific 160 nucleotide RNA molecule. It is the aim of the proposed studies to: I) Expand the information about the collection of brain cDNA clones to get a better overview of brain gene expression, II) Construct and investigate a similar random cDNA collection from another tissue (kidney) in order to separate the features of brain specificity from features of general tissue specificity, III) Determine the reason that brain specific mRNAs are nearly twice the length of unregulated mRNAs, IV) Investigate families of coordinately controlled mRNAs, V) Establish critically or disprove a relationship between ID sequences and genes expressed in brain, VI) Study the transcription pattern of ID sequences in brain and other tissues, and VII) Investigate the structure, origin and possible function of the 160 nucleotide brain specific RNA.